Lecture Outline Chapter 16 A Universe of Galaxies

Lecture Outline Chapter 16: A Universe of Galaxies © 2015 Pearson Education, Inc.

16. 1 Islands of Stars Our goals for learning: • What are three major types of galaxies? • How are galaxies grouped together? © 2015 Pearson Education, Inc.

Hubble Deep Field • Our deepest images of the universe show a great variety of galaxies, some of them billions of light-years away. © 2015 Pearson Education, Inc.

Galaxies and Cosmology • A galaxy's age, its distance, and the age of the universe are all closely related. • The study of galaxies is thus intimately connected with cosmology—the study of the structure and evolution of the universe. © 2015 Pearson Education, Inc.

What are three major types of galaxies? © 2015 Pearson Education, Inc.

Hubble Ultra Deep Field © 2015 Pearson Education, Inc.

Hubble Ultra Deep Field © 2015 Pearson Education, Inc.

Hubble Ultra Deep Field © 2015 Pearson Education, Inc.

Hubble Ultra Deep Field © 2015 Pearson Education, Inc.

Hubble Ultra Deep Field © 2015 Pearson Education, Inc.

Hubble Ultra Deep Field © 2015 Pearson Education, Inc.

Hubble Ultra Deep Field © 2015 Pearson Education, Inc.

Spiral galaxy © 2015 Pearson Education, Inc.

Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge and halo, old stars, few gas clouds © 2015 Pearson Education, Inc.

Disk Component: stars of all ages, many gas clouds Spheroidal Component: bulge and halo, old stars, few gas clouds © 2015 Pearson Education, Inc.

Disk Component: stars of all ages, many gas clouds Blue-white color indicates ongoing star formation. Spheroidal Component: bulge and halo, old stars, few gas clouds Red-yellow color indicates older star population. © 2015 Pearson Education, Inc.

Thought Question Why does ongoing star formation lead to a blue-white appearance? A. There aren't any red or yellow stars. B. Short-lived blue stars outshine others. C. Gas in the disk scatters blue light. © 2015 Pearson Education, Inc.

Thought Question Why does ongoing star formation lead to a blue-white appearance? A. There aren't any red or yellow stars. B. Short-lived blue stars outshine others. C. Gas in the disk scatters blue light. © 2015 Pearson Education, Inc.

Barred Spiral Galaxy: Has a bar of stars across the bulge. © 2015 Pearson Education, Inc.

Lenticular Galaxy: Has a disk like a spiral galaxy but much less dusty gas (intermediate between spiral and elliptical). © 2015 Pearson Education, Inc.

Elliptical Galaxy: All spheroidal component, virtually no disk component © 2015 Pearson Education, Inc.

Elliptical Galaxy: All spheroidal component, virtually no disk component Red-yellow color indicates older star population. © 2015 Pearson Education, Inc.

Irregular Galaxy: Neither spiral nor elliptical. Bluewhite color indicates ongoing star formation. © 2015 Pearson Education, Inc.

Spheroid dominates © 2015 Pearson Education, Inc. Hubble's galaxy classes Disk dominates

How are galaxies grouped together? © 2015 Pearson Education, Inc.

Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies per group). © 2015 Pearson Education, Inc.

Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies). © 2015 Pearson Education, Inc.

What have we learned? • What are three major types of galaxies? – Spiral galaxies, elliptical galaxies, and irregular galaxies – Spirals have both disk and spheroidal components; ellipticals have no disk. • How are galaxies grouped together? – Spiral galaxies tend to collect into groups of up to a few dozen galaxies. – Elliptical galaxies are more common in large clusters containing hundreds to thousands of galaxies. © 2015 Pearson Education, Inc.

16. 2 Distances of Galaxies Our goals for learning: • How do we measure the distances to galaxies? • What is Hubble's law? • How do distance measurements tell us the age of the universe? © 2015 Pearson Education, Inc.

How do we measure the distances to galaxies? © 2015 Pearson Education, Inc.

Brightness alone does not provide enough information to measure distance. Are Bright Stars Nearby or Luminous? © 2015 Pearson Education, Inc.

Step 1 Determine size of solar system using radar. Radar Pulses © 2015 Pearson Education, Inc.

Step 2 Determine distances of stars out to a few hundred light-years using parallax. © 2015 Pearson Education, Inc.

Luminosity passing through each sphere is the same. Area of sphere: 4 (radius)2 Divide luminosity by area to get brightness. © 2015 Pearson Education, Inc.

The relationship between apparent brightness and luminosity depends on distance. Brightness = Luminosity 4 (distance)2 We can determine a star's distance if we know its luminosity and can measure its apparent brightness. Distance = Luminosity 4 × Brightness A standard candle is an object whose luminosity we can determine without measuring its distance. © 2015 Pearson Education, Inc.

Step 3 Apparent brightness of star cluster's main sequence tells us its distance. © 2015 Pearson Education, Inc.

Knowing a star cluster's distance, we can determine the luminosity of each type of star within it. © 2015 Pearson Education, Inc.

Thought Question Which kind of stars are best for measuring large distances? A. High-luminosity stars B. Low-luminosity stars © 2015 Pearson Education, Inc.

Thought Question Which kind of stars are best for measuring large distances? A. High-luminosity stars B. Low-luminosity stars © 2015 Pearson Education, Inc.

Cepheid variable stars are very luminous. © 2015 Pearson Education, Inc.

Cepheid Variable Stars The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50 -day period. © 2015 Pearson Education, Inc.

Cepheid variable stars with longer periods have greater luminosities. © 2015 Pearson Education, Inc.

Step 4 Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles. Using Cepheid Variables as Standard Candles © 2015 Pearson Education, Inc.

White dwarf supernovae can also be used as standard candles. © 2015 Pearson Education, Inc.

Step 5 Apparent brightness of a white dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years). © 2015 Pearson Education, Inc.

What is Hubble's law? © 2015 Pearson Education, Inc.

The Puzzle of "Spiral Nebulae" • Before Hubble, some scientists argued that "spiral nebulae" were entire galaxies like our Milky Way, whereas other scientists maintained they were smaller collections of stars within the Milky Way. • The debate remained unsettled until someone finally measured the distances of spiral nebulae. © 2015 Pearson Education, Inc.

Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles. © 2015 Pearson Education, Inc.

Hubble also knew that the spectral features of virtually all galaxies are redshifted ⇒ they're all moving away from us. © 2015 Pearson Education, Inc.

By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way. Discovering Hubble's Law © 2015 Pearson Education, Inc.

© 2015 Pearson Education, Inc. Hubble's law: velocity = H 0 × distance

Redshift of a galaxy tells us its distance through Hubble's law: velocity distance = H 0 © 2015 Pearson Education, Inc.

Distances of the farthest galaxies are measured from redshifts. © 2015 Pearson Education, Inc.

We measure galaxy distances using a chain of interdependent techniques. © 2015 Pearson Education, Inc.

How do distance measurements tell us the age of the universe? © 2015 Pearson Education, Inc.

Thought Question Your friend leaves your house. She later calls you on her cell phone, saying that she's been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone? A. 1 minute B. 30 minutes C. 60 minutes D. 120 minutes © 2015 Pearson Education, Inc.

Thought Question Your friend leaves your house. She later calls you on her cell phone, saying that she's been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone? A. 1 minute B. 30 minutes C. 60 minutes D. 120 minutes © 2015 Pearson Education, Inc.

Thought Question You observe a galaxy moving away from you at 0. 1 light-years per year, and it is now 1. 4 billion lightyears away from you. How long has it taken to get there? A. 1 million years B. 14 million years C. 10 billion years D. 14 billion years © 2015 Pearson Education, Inc.

Thought Question You observe a galaxy moving away from you at 0. 1 light-years per year, and it is now 1. 4 billion lightyears away from you. How long has it taken to get there? A. 1 million years B. 14 million years C. 10 billion years D. 14 billion years © 2015 Pearson Education, Inc.

Hubble's constant tells us the age of the universe because it relates velocities and distances of all galaxies. Distance Age = Velocity ~ 1 / H 0 Estimating the Age of the Universe © 2015 Pearson Education, Inc.

The expansion rate appears to be the same everywhere in space. The universe has no center and no edge (as far as we can tell). Two Possible Explanations of the Cause of Hubble's Law © 2015 Pearson Education, Inc.

One example of something that expands but has no center or edge is the surface of a balloon. © 2015 Pearson Education, Inc.

Cosmological Principle The universe looks about the same no matter where you are within it. • Matter is evenly distributed on very large scales in the universe. • No center and no edges • Not proved but consistent with all observations to date © 2015 Pearson Education, Inc.

Distances between faraway galaxies change while light travels. © 2015 Pearson Education, Inc.

Distances between faraway galaxies change while light travels. distance? © 2015 Pearson Education, Inc. Astronomers think in terms of lookback time rather than distance.

Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time. © 2015 Pearson Education, Inc.

What have we learned? • How do we measure the distances to galaxies? – The distance measurement chain begins with parallax measurements that build on radar ranging in our solar system. – Using parallax and the relationship between luminosity, distance, and brightness, we can calibrate a series of standard candles. – We can measure distances greater than 10 billion light-years using white dwarf supernovae as standard candles. © 2015 Pearson Education, Inc.

What have we learned? • What is Hubble's law? – The faster a galaxy is moving away from us, the greater its distance: velocity = H 0 × distance © 2015 Pearson Education, Inc.

What have we learned? • How do distance measurements tell us the age of the universe? – Measuring a galaxy's distance and speed allows us to figure out how long the galaxy took to reach its current distance. – Measuring Hubble's constant tells us that amount of time: about 14 billion years. © 2015 Pearson Education, Inc.

16. 3 Galaxy Evolution Our goals for learning: • How do we study galaxy evolution? • Why do galaxies differ? © 2015 Pearson Education, Inc.

How do we study galaxy evolution? © 2015 Pearson Education, Inc.

Deep observations show us very distant galaxies as they were much earlier in time (old light from young galaxies). © 2015 Pearson Education, Inc.

© 2015 Pearson Education, Inc.

Modeling Galaxy Formation: • Matter originally filled all of space almost uniformly. • Gravity of denser regions pulled in surrounding matter. © 2015 Pearson Education, Inc.

Denser regions contracted, forming protogalactic clouds. H and He gases in these clouds formed the first stars. © 2015 Pearson Education, Inc.

Supernova explosions from the first stars kept much of the gas from forming stars. Leftover gas settled into a spinning disk. Conservation of angular momentum © 2015 Pearson Education, Inc.

M 101 M 87 But why do some galaxies end up looking so different? © 2015 Pearson Education, Inc.

Why do galaxies differ? © 2015 Pearson Education, Inc.

Why don't all galaxies have similar disks? © 2015 Pearson Education, Inc.

Conditions in Protogalactic Cloud? Spin: Initial angular momentum of protogalactic cloud could determine the size of the resulting disk. © 2015 Pearson Education, Inc.

Conditions in Protogalactic Cloud? Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk. © 2015 Pearson Education, Inc.

Distant Red Ellipticals Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe. © 2015 Pearson Education, Inc.

We must also consider the effects of collisions. © 2015 Pearson Education, Inc.

Collisions were much more likely early in time, because galaxies were closer together. © 2015 Pearson Education, Inc.

Many of the galaxies we see at great distances (and early times) do indeed look violently disturbed. © 2015 Pearson Education, Inc.

The collisions we observe nearby trigger bursts of star formation. © 2015 Pearson Education, Inc.

Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical. © 2015 Pearson Education, Inc.

Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together. © 2015 Pearson Education, Inc.

Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies. © 2015 Pearson Education, Inc.

Starburst galaxies are forming stars so quickly that they will use up all their gas in less than a billion years. © 2015 Pearson Education, Inc.

The intensity of supernova explosions in starburst galaxies can drive galactic winds. © 2015 Pearson Education, Inc.

X-ray image The intensity of supernova explosions in starburst galaxies can drive galactic winds. © 2015 Pearson Education, Inc.

What have we learned? • How do we study galaxy evolution? – Deep observations of the universe are showing us the history of galaxies because we are seeing galaxies as they were at different ages. – We use computer modeling to match theory with observation. © 2015 Pearson Education, Inc.

What have we learned? • Why do galaxies differ? – Some of the differences between galaxies may arise from the conditions in their protogalactic clouds. – Collisions can also play a major role because they can transform two spiral galaxies into an elliptical galaxy. © 2015 Pearson Education, Inc.

16. 4 Active Galactic Nuclei Our goals for learning: • What is the evidence for supermassive black holes at the centers of galaxies? • Why do we think the growth of central black holes is related to galaxy evolution? © 2015 Pearson Education, Inc.

What is the evidence for supermassive black holes at the centers of galaxies? © 2015 Pearson Education, Inc.

If the center of a galaxy is unusually bright, we call it an active galactic nucleus. The most luminous examples are called quasars. Active Nucleus in M 87 © 2015 Pearson Education, Inc.

The highly redshifted spectra of quasars indicate large distances. From brightness and distance, we find that luminosities of some quasars are >1012 LSun! Variability shows that all this energy comes from a region smaller than the solar system. © 2015 Pearson Education, Inc.

Thought Question What can you conclude from the fact that quasars usually have very large redshifts? A. They are generally very distant. B. They were more common early in time. C. Galaxy collisions might turn them on. D. Nearby galaxies might hold dead quasars. © 2015 Pearson Education, Inc.

Thought Question What can you conclude from the fact that quasars usually have very large redshifts? A. They are generally very distant. B. They were more common early in time. C. Galaxy collisions might turn them on. D. Nearby galaxies might hold dead quasars. All of the above! © 2015 Pearson Education, Inc.

Galaxies around quasars sometimes appear disturbed by collisions. © 2015 Pearson Education, Inc.

Quasars powerfully radiate energy over a very wide range of wavelengths, indicating that they contain matter with a wide range of temperatures. © 2015 Pearson Education, Inc.

Radio galaxies contain active nuclei shooting out vast jets of plasma, which emit radio waves coming from electrons moving at near light speed. © 2015 Pearson Education, Inc.

The lobes of radio galaxies can extend over hundreds of millions of light-years. © 2015 Pearson Education, Inc.

An active galactic nucleus can shoot out blobs of plasma moving at nearly the speed of light. The speed of ejection suggests that a black hole is present. © 2015 Pearson Education, Inc.

Radio galaxies don't appear as quasars because dusty gas clouds block our view of their accretion disks. © 2015 Pearson Education, Inc.

Characteristics of Active Galaxies • Luminosity can be enormous (>1012 LSun). • Luminosity can vary rapidly (comes from a space smaller than solar system). • They emit energy over a wide range of wavelengths (contain matter with wide temperature range). • Some drive jets of plasma at near light speed. © 2015 Pearson Education, Inc.

The accretion of gas onto a supermassive black hole appears to be the only way to explain all the properties of quasars. © 2015 Pearson Education, Inc.

Energy from a Black Hole • The gravitational potential energy of matter falling into a black hole turns into kinetic energy. • Friction in the accretion disk turns kinetic energy into thermal energy (heat). • Heat produces thermal radiation (photons). • This process can convert 10– 40% of E = mc 2 into radiation. © 2015 Pearson Education, Inc.

Jets are thought to come from the twisting of a magnetic field in the inner part of the accretion disk. © 2015 Pearson Education, Inc.

Why do we think the growth of central black holes is related to galaxy evolution? © 2015 Pearson Education, Inc.

Orbits of stars at center of Milky Way indicate a black hole with mass of 4 million Msun. © 2015 Pearson Education, Inc.

Orbital speed and distance of gas orbiting center of M 87 indicate a black hole with mass of at least 3 billion Msun. © 2015 Pearson Education, Inc.

Black Holes in Galaxies • Many nearby galaxies—perhaps all of them—have supermassive black holes at their centers. • These black holes seem to be dormant active galactic nuclei. • All galaxies may have passed through a quasarlike stage earlier in time. © 2015 Pearson Education, Inc.

Galaxies and Black Holes • The mass of a galaxy's central black hole is closely related to the mass of its bulge. © 2015 Pearson Education, Inc.

Galaxies and Black Holes • The development of a central black hole must somehow be related to galaxy evolution. © 2015 Pearson Education, Inc.

What have we learned? • What is the evidence for supermassive black holes at the centers of galaxies? – Active galactic nuclei are very bright objects seen in the centers of some galaxies, and quasars are the most luminous type. – The only model that adequately explains our observations holds that supermassive black holes are the power source. © 2015 Pearson Education, Inc.

What have we learned? • Why do we think the growth of central black holes is related to galaxy evolution? – Observations of stars and gas clouds orbiting the centers of galaxies indicate that many galaxies, perhaps all of them, have supermassive black holes. – The masses of the black holes are closely related to the properties of their home galaxies, suggesting a connection between the black hole and galaxy evolution. © 2015 Pearson Education, Inc.